1. Field of the Invention
The present invention relates to a phosphate-treated steel plate with a substrate of zinc-base plated steel plate, which is used for body plates of automobiles and for household electric appliances, and the like.
2. Description of the Related Arts
Main stream of surface-treated steel plates for body plates of automobiles in prior art was electrolytic Znxe2x80x94Ni alloy plated steel plates, organic composite coating steel plates (electrolytic Znxe2x80x94Ni alloy plating+chromate coating+organic coating), and alloyed hot dip galvanized steel plates. In recent years, automobile manufacturers have tried to use inexpensive 100% zinc plated steel plates as the body plates of automobiles in view of cost reduction. The 100% zinc plated steel plates have, however, a plating layer in soft and with low melting point, so that the 100% zinc plated steel plates tend to induce fusion between plating layer and tool during press-forming, which raises a problem of easy occurrence of press-crack at portions of complex shapes. Accordingly, development of materials having superior lubrication performance was waited. In this regard, there have been introduced responding technologies given below.
(1) JP-A-7-138764, (the term xe2x80x9cJP-A-xe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), (hereinafter referred to as the xe2x80x9cPrior Art 1xe2x80x9d), discloses a zinc phosphate-treated steel plate which comprises: a zinc-containing metal plated steel plate; a zinc phosphate coating layer having a specified weight ratio of zinc to phosphorus and having a specified weight ratio of a specified metal, formed on the zinc-containing metal plated plate; and a lubricant oil layer on the zinc phosphate coating layer.
(2) JP-A-9-049086, (hereinafter referred to as the xe2x80x9cPrior Art 2xe2x80x9d), discloses a method for manufacturing an electroplated steel plate having high whiteness degree and excellent coatability, which method comprises the step of treating an electrolytically galvanized steel plate using a treatment solution containing specified amount of phosphoric acid ion, zinc ion, magnesium ion, nickel ion, and other ions, under a specified condition.
The zinc phosphate-treated steel plates which are disclosed in above-given Prior Arts 1 and 2 show an improvement in lubrication to some degree. The improvement effect is, however, not a satisfactory level. Furthermore, the zinc phosphate coating on these steel plates has a porous structure, so that the steel plates show poor corrosion resistance at portions where the electrodeposition coating cannot fully cover and where substrate steel plate is likely left exposed even after the electrodeposition coating, which portions include flange section and hem section observed at joints of body plates of automobiles. In addition, the electroplated steel plates which are manufactured by the technology disclosed in the Prior Art 2 give not satisfactory level of coating adhesiveness and of post-coating corrosion resistance in the case of two or more coating layers which are applied to the steel plates for body plates of automobiles.
As for the technology to improve the corrosion resistance of zinc phosphate-treated steel plates, prior art in the household electric appliances applied the zinc phosphate treatment followed by sealing treatment using a chromate-base aqueous solution. The conventional sealing technology for the zinc phosphate-treated steel plates uses hexavalent chromium which is a substance under environmental regulations. Therefore, technology which does not use chromium was wanted. Responding to the need, the following-listed technologies have been proposed.
(3) JP-A-56-136979, (hereinafter referred to as the xe2x80x9cPrior Art 3xe2x80x9d), discloses a treatment method applying phosphate treatment to a cold-rolled steel plate or a galvanized steel plate, then immediately applying a post-treatment using a treatment solution consisting mainly of a chelating agent.
(4) JP-A-58-197284, (hereinafter referred to as the xe2x80x9cPrior Art 4xe2x80x9d), discloses a method of treament before coating for zinc-base plated steel plates, which method comprises the steps of: applying phosphate treatment to the zinc plated steel plates, then applying treatment using an aqueous solution containing a polyacrylic acid and a aromatic polyhydric alcohol.
(5) JP-B-63-4916, (the term xe2x80x9cJP-B-xe2x80x9d referred to herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), (hereinafter referred to as the xe2x80x9cPrior Art 5xe2x80x9d), discloses a composite plated steel plate having excellent durability, which steel plate comprises a steel plate, a Znxe2x80x94Ni alloy plating, a phosphoric acid coating at coating weights of from 1 to 2 g/m2 on the Znxe2x80x94Ni alloy plating, and a polymer coating having thicknesses of from 5 to 10 xcexcm on the phosphoric acid coating.
The above-described conventional zinc phosphate-treated steel plates have, however, problems given below. That is, the zinc phosphate-treated steel plates in the Prior Arts 3 and 4 use ordinary zinc phosphate coating, so that these steel plates have no coating adhesiveness that is required as the steel plates for automobiles. In addition, the organic sealing which is disclosed in these prior arts is dissolved or degraded owing to the contact with alkaline or acidic solution met in the process of automobile body assembly: [shearingxe2x86x92pressingxe2x86x92welding alkali degreasingxe2x86x92chemical conversion electrodeposition coatingxe2x86x92intermediate coating and top coating]. As a result, the corrosion resistance of these steel plates is poor.
The zinc phosphate-treated steel plate of the Prior Art 5 uses ordinary zinc phosphate coating, similar with that of the Prior Arts 3 and 4, so that the steel plate has no coating adhesiveness that is required as the steel plates for automobiles. In addition, since the organic coating is very thin, 5 to 10 xcexcm, the spot welding is very difficult, and the coating is easily peeled during the press-forming stage owing to the bending and unbending at bead portions, (resulting in poor anti-powdering performance), further the peeled coating degrades the lubricant performance, which results in poor press-formability.
Consequently, prior arts fail to satisfy all of the required performance of: corrosion resistance, anti-powdering performance, lubrication, coating adhesiveness, and weldability.
An object of the present invention is to provide an environmentally friendly surface-treated steel plate which has excellent corrosion resistance, anti-powdering performance, lubrication, coating adhesiveness, and weldability, and which contains no chromium.
To achieve the above-given object, the present invention provides a phosphate-treated steel plate which comprises: a zinc-base plated steel plate; a zinc phosphate coating layer formed on the surface of the zinc-base plated steel plate; and an organic coating formed on the zinc phosphate coating layer.
The zinc phosphate coating layer contains at least one substance selected from the group consisting of nickel, manganese, and magnesium, at coating weights of from 0.2 to 2.5 g/m2.
The organic coating consists of at least one organic resin selected from the group consisting of an ethylene-base resin, an epoxy-base resin, a urethane-base resin, and an acrylic-base resin.
The epoxy-base resin is preferably a block urethane-modified resin prepared by mixing a modified epoxy resin (A) comprising an epoxy resin, a multifunctional amine, and a monoisocyanate, and a block urethane (B) comprising a polyol, a polyisocyanate, and a block agent, at mixing rates (A/B) of from 95/5 to 50/50 (weight ratio of nonvolatile matter).
The epoxy-base resin is preferably an epoxy-base resin prepared by mixing 5 to 80 parts by weight (solid content) of a polyisocyanate compound having at least two isocyanate groups in a single molecule thereof, and 100 parts by weight (solid content) of a substrate resin in which at least one basic nitrogen atom and at least two primary hydroxide groups are added to a terminal of the molecular chain of the epoxy resin.
The present invention provides a phosphate-treated steel plate which comprises: a zinc-base plated steel plate; a zinc phosphate coating formed on the zinc-base plated steel plate; and a phosphate coating formed on the zinc phosphate coating.
The zinc phosphate coating consists mainly of zinc phosphate. The phosphate coating consists mainly of a phosphate of at least one metal selected from the group consisting of Mg, Al, Co, Mn, and Ca.
Preferred Embodiment 1
The inventors of the present invention investigated the zinc phosphate composite treated steel plates focusing on the relation of coating in terms of structure, corrosion resistance, anti-powdering performance, lubrication, coating adhesiveness, and weldability. Thus, the investigation derived the following-described findings.
(1) As for the improvement in corrosion resistance, coating adhesiveness, lubrication, and anti-powdering performance, it is effective to optimize the composition of zinc phosphate which forms the first layer, and to establish a dual-layer structure which comprises the zinc phosphate coating and an organic coating consisting mainly of a specified organic resin as the top layer for sealing.
(2) The corrosion resistance is further improved by adding a specified rust-preventive additive at a specified amount to the organic coating, without degrading the lubrication, the coating adhesiveness, and the weldability.
(3) The lubrication and the anti-powdering performance are further improved by adding a specified lubricant at a specified amount to the organic coating, without degrading the coating adhesiveness and the weldability.
(4) The lubrication, the corrosion resistance, the coating adhesiveness, the weldability, and the anti-powdering performance are improved by optimizing the coating weight of the zinc phosphate coating as the first layer and of the organic coating as the second layer.
The present invention was established on the basis of above-described findings, and the present invention is characterized in the constitution described in the following.
That is, the present invention provides a zinc phosphate composite treated steel plate having excellent corrosion resistance, anti-powdering performance, lubrication, and coating adhesiveness, which steel plate comprises: a zinc-base plated steel plate; a first layer of zinc phosphate coating layer having coating weights of from 0.2 to 2.5 g/m2, containing at least one substance selected from the group consisting of nickel, manganese, and magnesium, formed on the surface of the zinc-base plated steel plate; and a second layer of an organic coating consisting mainly of at least one organic resin selected from the group consisting of an ethylene-base resin, an epoxy-base resin, a urethane-base resin, and an acrylic-base resin, formed on the zinc phosphate coating layer.
The content of at least one substance selected from the group consisting of nickel, manganese, and magnesium, in the zinc phosphate coating is preferably in a range of from 0.5 to 8.5 mass % as the total thereof.
The organic coating as the second layer preferably contains a solid lubricant and/or a rust-preventive additive as components other than the organic resin.
The rust-preventive additive is preferably at least one substance selected from the group consisting of a silica and a phosphate. The silica is preferably at least one substance selected from the group consisting of ion-exchanged silica, fumed silica, and colloidal silica. Furthermore, the ion-exchanged silica is preferably Ca-exchanged silica.
The phosphate is preferably at least one substance selected from the group consisting of a phosphate of calcium, aluminum, and zinc.
The solid lubricant is preferably at least one substance selected from the group consisting of polyethylene wax, tetrafluoroethylene resin, and boron nitride. The average particle size of the solid lubricant is preferably in a range of from 0.05 to 25 xcexcm. The polyethylene wax preferably has a softening point in a range of from 100 to 135xc2x0 C.
The content of the rust-preventive additive in the organic coating is preferably in a range of from 1 to 100 parts by weight as solid content to 100 parts by weight of the organic resin. And the content of the solid lubricant is preferably in a range of from 1 to 80 parts by weight as solid content to 100 parts by weight of the organic resin.
The coating weight of the organic coating film is preferably in a range of from 0.05 to 1.5 g/m2.
As the uppermost layer, a rust-preventive oil film layer as the third layer is preferably formed at coating weights of from 0.01 to 10 g/m2.
The surface-treated steel plates according to the present invention are applicable not only to automobiles and household electric appliances, but also to building materials and the like.
The detail of the present invention is described in the following giving the reasons to limit the specification.
The steel plates which become the substrate of the zinc-base plated steel plates according to the present invention include: all kinds of cold-rolled steel plates for soft-working, such as cold-rolled steel plates for general working (CQ), cold-rolled steel plates for deep drawing (DQ), cold-rolled steel plates for very deep drawing (DDQ), and cold-rolled steel plates for ultra deep drawing (EDDQ); all kinds of high tension steel plates ranging from high tension steel plates of relatively low strength level having baking-hardening property to general high tension steel plates having more than 390 MPa of tensions; and de-scaled hot-rolled steel plates.
Examples of the plating layers of the zinc-base plated steel plates are Zn plating, Znxe2x80x94Ni alloy plating (10 to 15 mass % of Ni content), Znxe2x80x94Fe ally plating (5 to 25 mass % or 60 to 90 mass % of Fe content), Znxe2x80x94Mn alloy plating (30 to 80 mass % of Mn content), Znxe2x80x94Co alloy plating (0.5 to 15 mass % of Co), Znxe2x80x94Cr ally plating (5 to 30 mass % of Cr), Znxe2x80x94Al alloy plating (3 to 60 mass % of Al content). Each of the above-given plating compositions may further include alloying element such as Co, Fe, Ni, and Cr, and oxide or salt of silica, alumina, slightly soluble chromate, or the like, and polymer. Among the above-described plating layers, two or more layers of the same kind or different kind may be applied to form a composite layer.
The plated steel plate may be the one prepared by applying plating of Ni or the like at a small coating weight onto the steel plate, followed by applying various kinds of plating given above.
The various kinds of plating described above may be formed by either one of electrolytic method, fusion method, and vapor phase method. A preferred coating weight of plating is not less than 10 g/m2. Less than 10 g/m2 of coating weight induces problems because of poor corrosion resistance. In the case of Znxe2x80x94Ni alloy plating, Znxe2x80x94Fe alloy plating, Znxe2x80x94Mn alloy plating, Znxe2x80x94Co alloy plating, and Znxe2x80x94Cr alloy plating, the anti-powdering performance degrades when the coating weight exceeds 60 g/m2, so the coating weight is preferably in a range of from 10 to 60 g/m2. For further improved corrosion resistance and anti-powdering performance, the coating weight is preferably in a range of from 15 to 60 g/m2.
To prevent generation of film defects and irregularity on the surface of the zinc phosphate composite coating on the plating film, which processing is described later, it is possible to apply treatment of alkaline degreasing, solvent degreasing, and surface preparation treatment on the plating film, in advance. These pre-treatments include (1) the treatment using an acidic or alkaline aqueous solution containing at least one metallic ion selected from the group consisting of Ni ion, Co ion, and Fe ion, (2) the treatment contacting with a titanium colloid aqueous solution, and (3) the treatment to etch the top layer of the metallic oxide formed on the surface of the plated steel plate using an inorganic acid, an organic acid, or a cheleting compound such as EDTA and NTA. The effect of the present invention is available with any of these kinds of steel plates as the substrate.
On the above-described zinc-base plated steel plates, a zinc phosphate coating is formed as the first layer, and an organic coating is formed as the second layer on the first layer. The zinc-phosphate coating of the first layer improves the coating adhesiveness owing to the anchor effect, and contributes to the improvement of lubrication by preventing the direct contact between the steel plate and the tools during sliding actions.
According to the present invention, a zinc phosphate coating containing at least one substance selected from the group consisting of nickel, manganese, and magnesium is applied. The coating exists presumably in a form that a portion of zinc in the zinc phosphate coating is substituted by the above-described metal contained in the coating. That form of coating induces the interaction with the organic coating as the top layer, thus providing excellent corrosion resistance, anti-powdering performance, lubrication, and coating adhesiveness.
The content of at least one substance selected from the group consisting of nickel, manganese, and magnesium, in the zinc phosphate coating is preferably in a range of from 0.5 to 8.5 mass % as the total. By specifying the total content of these metals in the coating in that range, the corrosion resistance, the lubrication, and the coating adhesiveness are further improved. When particularly superior corrosion resistance and coating adhesiveness are required, it is more preferable to specify the total content of nickel, manganese, and magnesium to a range of from 3 to 6 mass %. The corrosion resistance and the coating adhesiveness are drastically improved by the coexistence of nickel and manganese, nickel and magnesium, or nickel and manganese and magnesium, in the zinc phosphate coating.
The coating weight of the zinc phosphate coating as the first layer is preferably in a range of from 0.2 to 2.5 g/m2. If the coating weight thereof is less than 0.2 g/m2, the coating adhesiveness and the corrosion resistance degrade. If the coating weight thereof exceeds 2.5 g/m2, the spot weldability degrades, and the powdering under sliding condition increases, and the lubrication also degrades. In view of lubrication, coating adhesiveness, corrosion resistance, and weldability, more preferable range of coating weight is from 0.5 to 1.5 g/m2, and most preferably from 0.5 to 1.0 g/m2. The method of zinc phosphate treatment for forming the zinc phosphate coating may be either one of reaction type treatment, coating type treatment, and electrolytic type treatment.
An example of the reaction type treatment is that a plated steel plate is subjected to degreasing, washing with water, and surface preparation treatment, followed by contacting with a treatment solution of an aqueous solution consisting mainly of: phosphoric acid ion, nitric acid ion, and zinc ion, and at least one substance selected from the group consisting of nickel ion, manganese ion, and magnesium ion; further containing, at need, (1) and (2) given below, then washing with water and drying.
(1) At least one substance selected from the group consisting of iron ion, cobalt ion, and calcium ion.
(2) At least one substance selected from the group consisting of peroxide, fluoride ion, fluorine complex ion, and nitrous acid ion.
Regarding the coating type treatment, at least one side of the plated steel plate is coated with a zinc phosphate treatment solution consisting mainly of phosphoric acid ion, nitric acid ion, and zinc ion, and at least one substance selected from the group consisting of nickel ion, manganese ion, and magnesium ion. Any kind of coating method is applicable. That is, coating by roll-coater method, coating by immersion method or spray method followed by applying air-knife method or roll-squeezing method to adjust the coating weight may be used. After coating a zinc phosphate treatment solution onto the surface of the plated steel plate, drying may be given using a drier, a hot air furnace, a high frequency induction heating furnace, or an infrared furnace to form the zinc phosphate coating.
The drying temperature is preferably in a range of from 70 to 400xc2x0 C. as the ultimate plate temperature. If the drying temperature is less than 70xc2x0 C., the drying of coating becomes insufficient, which induces stickiness of the coating and degradation in coating adhesiveness, and induces irregular coating on forming the organic coating of the second layer. If the ultimate plate temperature exceeds 400xc2x0 C., the effect saturates, which not only is uneconomical but also degrades corrosion resistance owing to the tendency of defect occurrence in coating. Accordingly, more preferable baking temperature is in a range of from 100 to 300xc2x0 C., and most preferable one is from 120 to 170xc2x0 C.
The organic coating formed as the second layer on the zinc phosphate coating is described below. According to the present invention, the organic coating formed on the above-described zinc phosphate coating is an organic coating consisting mainly of at least one organic resin selected from the group consisting of an ethylene-base resin, an epoxy-base resin, a urethane-base resin, and an acrylic-base resin. By using these resins, the favorable coating adhesiveness and corrosion resistance are attained. Examples of these resins are the following.
Examples of the ethylene-base resin are: an ethylene-base copolymer such as ethylene-acrylic acid copolymer, ethyelen-methacrylic acid copolymer, and carboxyl-modified polyolefin resin; an ethylene-unsaturated carboxylic acid copolymer; an ethylene-base ionomer; and resins prepared by modifying those resins with alkyd resin, epoxy resin, phenolic resin, and the like.
Examples of the epoxy resin are: aromatic epoxy resins which are prepared either by introducing glycidyl group through the reaction between a polyphenol such as Bisphenol A, Bisphenol B, Bisphenol F, and novorak type phenol and an epihalohydrin such as epichlorohydrin, or by increasing their molecular weight through further reaction between the product of glycidyl group-introduction reaction and a polyphenol; aliphatic epoxy resins; and alicyclic epoxy resins. Among these epoxy resins, perticularly when film-forming is required at a low temperature, the epoxy resins having not less than 1,500 of average molecular weight are preferred.
In addition, resins prepared by reacting various kinds of modifiers with the epoxy group or the hydroxyl group in the above-described epoxy resins may be applied. Examples of these resins are: an epoxy-ester resin prepared by reacting with a drying oil fatty acid; an epoxy-acrylate resin prepared by modifying using a polymerizable unsaturated monomer component containing acrylic acid, methacrylic acid, and the like; a urethane-modified epoxy resin prepared by reacting with an isocyanate compound; a polybasic acid-modified epoxy resin; an acrylic resin-modified epoxy resin; an alkyd (or polyester)-modified epoxy resin; a polybutadiene-modified epoxy resin; a phenol-modified epoxy resin; and an amine or polyamine-modified epoxy resin.
Examples of the acrylic-base resin are: polyacrylic acid and its copolymer; polyacrylic acid ester and its copolymer; polymethacrylic acid and its copolymer; polymethacrylic acid ester and its copolymer; urethane-acrylic acid copolymer (or urethane-modified acrylic resin); styrene-acrylic acid copolymer; and resins prepared by modifying those resins with other alkyd resin, epoxy resin, phenol resin, and the like.
Examples of the urethane-base resin are: a polycarbonate-base polyurethane resin; a polyester-base polyurethane resin; and a polyether-base polyurethane resin.
According to the present invention, the above-described organic resins may be applied separately or mixing two or more of them. When particularly superior coating adhesiveness and corrosion resistance are required, it is preferred to use an epoxy-base resin, an ethylene-base resin, or an acrylic-base resin. These organic resins may be either one of water-soluble type, water-dispersing type, organic solvent-soluble type, and organic solvent-dispersing type.
According to the present invention, the organic coating may include a rust-preventive additive or a solid lubricant, or both of them, at need.
When particularly superior corrosion resistance is required, the addition of a rust-preventive additive is effective. Examples of preferred rust-preventive additive according to the present invention are a silica, a phosphate, a molybdate, a phosphomolybdate (for example, aluminum phosphomolybdate), an organic phosphoric acid and its salt (for example, phytic acid, phosphonic acid, and their metallic salt, alkali metal salt, alkali earth metallic salt); an organic inhibitor (for example, hydrazine derivative, thiol compound). These rust-preventive additives may be used separately or mixing two or more of them.
Among these rust-preventive additives, silica and phosphate are more preferable. Examples of the silica are ion-exchanged silica prepared by fixing a metallic ion of calcium, magnesium, and the like, onto the surface of the porous silica gel powder; fumed silica; colloidal silica; and organosilica sol. These silicas may be used separately or two or more of them together. Among these silicas, more preferable ones are the ion-exchanged silica, the fumed silica having primary particle sizes of from 5 to 50 nm, and the colloidal silica, and most preferable one is the calcium ion-exchanged silica having 1 mass % or more of calcium concentration.
The phosphate according to the present invention is not limited by the skeleton and the degree of condensation of the phosphoric acid ions, and it may be either one of normal salt, dihydrogen salt, monohydrogen salt, and phosphite. The normal salt includes orthophosphate, all kinds of condensed phosphate such as polyphosphate (for example, zinc phosphate, calcium phosphate, aluminum dihydrogen phosphate, zinc phosphate). Among them, more preferable ones are at least one phosphate selected from the group consisting of phosphate of zinc, of calcium, and of aluminum. Use of above-given silica and phosphate together provides particularly superior corrosion resistance.
According to the present invention, mixing a solid lubricant in the organic coating provides further superior lubrication performance. Examples of the solid lubricant preferred in the present invention are the following.
(1) Polyolefin wax, paraffin wax: for example, polyethylene wax, synthesized paraffin, micro wax, chlorinated hydrocarbon.
(2) Fluororesin-base wax: for example, polyfluoroethylene resin (polytetrafluoroethylene resin), polyfluorovynil resin, polyfluorovinylidene resin.
(3) Fatty acid amid-base compounds: for example, stearic acid amide, palmitic acid amide, methylene bis-stearoamide, ethylene bis-stearoamide, oleic acid amide, ethyl acid amide, alkylene bis-fatty acid amide.
(4) Metallic soaps: for example, calcium stearate, zinc stearate, calcium laurate, calcium palmitate.
(5) Metallic sulfides: for example, molybdenum disulfide, tungsten disulfide.
(6) Other: for example, graphite, graphite fluoride, boron nitride.
When particularly superior lubrication is required, it is preferable to use at least one compound selected from the group consisting of polyethylene wax, polytetrafluoroethylene resin, and boron nitride. Use of polyethylene wax and polytetrafluoroethylene resin together provides further superior lubrication performance.
The average particle size of the solid lubricant is preferably in a range of from 0.05 to 25 xcexcm. If the particle size is less than 0.05 xcexcm, the surface concentration of the lubricant is enriched to widen the occupied area of lubricant on the uppermost surface layer of the organic coating, which degrades the coating adhesiveness. On the other hand, if the particle size exceeds 25 xcexcm, the lubricant separates from the organic coating, which fails to attain the required lubrication, also results in poor corrosion resistance. To obtain excellent coating adhesiveness, corrosion resistance, lubrication, and anti-powdering performance, the average particle size is preferably in a range of from 1 to 15 xcexcm, and most preferably from 3 to 10 xcexcm. By regulating the softening point of polyethylene wax to a range of from 100 to 135xc2x0 C., more preferably from 110 to 130xc2x0 C., the lubrication and the anti-powdering performance are further improved.
A preferable content of lubricant and/or rust-preventive additive in the organic coating is in a range of from 1 to 100 parts by weight of the rust-preventive additive as solid content to 100 parts by weight of the organic resin, and in a range of from 1 to 80 parts by weight of the solid lubricant as solid content to 100 parts by weight of the organic resin.
If the content of the rust-preventive additive is less than 1 part by weight to 100 parts by weight of the organic resin, the improvement in corrosion resistance becomes insufficient. If the content of the rust-preventive additive exceeds 100 parts by weight to 100 parts by weight of the organic resin, the coating adhesiveness and the lubrication degrade. Accordingly, a preferable range of the content is from 10 to 80 parts by weight, most preferably from 20 to 70 parts by weight, in view of coating adhesiveness, lubrication, and corrosion resistance.
On the other hand, if the content of the solid lubricant is less than 1 part by weight to 100 parts by weight of the organic resin, the improvement effect of the lubrication is not sufficient. If the content exceeds 80 parts by weight, the coating adhesiveness and the corrosion resistance degrade. Thus, a preferable range of the content is from 3 to 50 parts by weight, and most preferably from 5 to 35 parts by weight, in view of coating adhesiveness, lubrication, and corrosion resistance.
The organic coating according to the present invention consists mainly of the above-described organic resin and, at need, the rust-preventive additive and/or the solid lubricant. Adding to those components, other components may further be added to the organic coating unless they do not give bad influence to the quality and performance of the organic coating. Examples of other applicable components are: an organic resin (for example; alkyd-base resin; fluorine-base resin; acrylic-silicone resin; silicone resin, phenol-base resin; melamine-base resin, amino-base resin); fine oxide particles such as those of alumina and zirconia; a conductive pigment; a color pigment (for example, condensed polycyclic organic pigment, phthalocyanine-base pigment); a color dye (for example, azo-base dye, azo-base metallic complex salt dye); a curing agent (for example, polyamine-base curing agent, acid anhydride curing agent, methylol group-contained initial condensate, polyisocyanate compound having at least two isocyanate groups in a single molecule); a film-forming assistant; a dispersion-improving agent; and a defoaming agent. These other components may be added separately or two or more thereof together.
A preferable range of coating weight of the organic coating is from 0.05 to 1.5 g/m2. If the coating weight is less than 0.05 g/m2, the corrosion resistance and the lubrication degrade. If the coating weight exceeds 1.5 g/m2, the weldability degrades. Thus, a preferable range of the coating weight is from 0.1 to 1.0 g/m2, and most preferably from 0.2 to 0.6 g/m2, in view of lubrication, corrosion resistance, coating adhesiveness, and weldability.
According to the present invention, the method for forming the organic coating comprises the steps of: applying a coating composition consisting mainly of the above-described organic resin and, at need, the above-described rust-preventive additive and/or the lubricant on to at least one side of the surfaces of the steel plate coated with the above-described zinc phosphate coating; drying the coating composition to form the coating. Before applying the coating composition, it is possible to arbitrarily give a preliminary treatment such as washing with water and drying the steel plate on which the zinc phosphate coating was formed.
Any type of method for applying the coating composition onto the steel plate may be adopted. Normally, the application is done by roll-coater method. However, it is possible to, after applying by immersion method and spray method, adjust the coating weight by air-knife method or roll-squeezing method.
The drying after applied the coating composition may be done by a drier, a hot-air furnace, a high frequency induction heating furnace, or an infrared furnace. A preferred drying temperature is in a range of from 50 to 300xc2x0 C. as the ultimate plate temperature. If the drying temperature is lower than 500xc2x0 C., the coating is insufficiently dried to induce stickiness on the coating, and the coating is damaged on touching to rolls after drying, which degrades the coating adhesiveness, the corrosion resistance, and the lubrication performance. If the ultimate plate temperature exceeds 300xc2x0 C., further effect cannot be expected, and the production cost becomes unfavorable. In this respect, a preferable range of baking temperature is from 100 to 200xc2x0 C., most preferably from 120 to 170xc2x0 C.
The present invention deals with a steel plate having the above-described coating structure on both sides or on one side thereof. Consequently, examples of the mode for carrying out the present invention are the following.
 
According to the present invention, the organic coating may further be covered with a rust-preventive oil layer as the third layer. The rust-preventive oil consists mainly of a rust-preventive additive (for example, oil-soluble surfactant), a petroleum-base base material (for example, mineral oil, solvent), an oil film adjuster (for example, mineral oil, crystallizing material, a viscous material), an antioxidizing agent (for example, phenol-base antioxidant), a lubricant (for example, extreme-pressure additive). Examples of the rust-preventive oil are a normal rust-preventive oil, a cleaning rust-preventive oil, a lubrication rust-preventive oil. Examples of the normal rust-preventive oil are a finger print removal type rust-preventive oil which is prepared by dissolving and decomposing a base material in a petroleum-base solvent, a solvent cutback type rust-preventive oil, a lubricant oil type rust-preventive oil using petrolactam and wax as the base materials, and a volatile rust-preventive oil.
A preferable coating weight of the rust-preventive oil film is in a range of from 0.01 to 10 g/m2. If the coating weight is less than 0.01 g/m2, the effect of rust-preventive oil application cannot be attained. If the coating weight exceeds 10 g/m2, the degreasing ends insufficiently, which results in poor coating adhesiveness. For attaining further superior corrosion resistance and coating adhesiveness, the coating weight is preferably in a range of from 0.5 to 3 g/m2.
Embodiments
Cold-rolled steel plates each having a plate thickness of 0.7 mm and a surface roughness (Ra) of 1.0 xcexcm were used to prepare plated steel plates by applying plating of zinc-base coating. Thus prepared plated steel plates were subjected to alkali degreasing, washing with water, and surface preparation treatment, then were brought into contact with a zinc phosphate treatment solution, followed by washing with water and drying, thus obtaining the zinc phosphate-treated steel plates. Onto the zinc phosphate-treated steel plates, respective coating compositions were applied using the roll coater method, which were then dried without washing with water. Then, a rust-preventive oil or a cleaning oil was applied to the dried steel plates. The obtained surface-treated steel plates were tested to determine lubrication performance, anti-powdering performance, coating adhesiveness, and weldability. Individual conditions are described below.